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About the Author:
My name is Cyprien Rusu, I am a French CAE
engineer who wants to teach the right bases of FEA
Simulation to designers, engineers and everyone
aspiring to get it right!
Hundreds of FEA students followed my free FEA
webinars on Youtube, read my blog articles on
feaforall.com and joined my FEA courses to learn
more and improve their understanding of FEA and
become better engineers!
I have also taught FEA seminars to FEA engineers from all over the
world…
Do you want to join my free FEA course?
Click on the link below and join the course to get a basic
understanding of the FEA foundations that you need to have:
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1- What is Fatigue Failure?
Fatigue failure is defined as the tendency of a material to fracture
by means of progressive brittle cracking under repeated
alternating or cyclic stresses of an intensity considerably below the
normal strength.
Although the fracture is of a brittle type, it may take some time to
propagate, depending on both the intensity and frequency of the
stress cycles.
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Examples of fatigue failure
All structures and mechanical components that are cyclically
loaded can fail by fatigue.
Fundamental requirements during design and manufacturing to
avoid fatigue failure are different for each different case and
should be considered during the design phase.
2- Fatigue Design Philosophy
Before attempting to carry out a fatigue calculation, or even
choosing a way of calculating, it is necessary in critical situations
to decide on a design philosophy.
The three main approaches are Safe-Life, Fail-Safe and Damage-
Tolerant.
Products are designed to survive a specific design
life. Full scale tests are usually carried out with
margins of safety applied. Fairly optimized structures.
A “safe-life” stool.
Any less than three legs and it would fall over!
Safe-Life
Fail-Safe Products are designed to avoid any failure regardless of
costs. If the structure were to fail in some part, it would
capable to working properly, until necessary repairs
could be made.
A “Fail-Safe” stool.
Failure of one leg would not result in overall failure.
Products are design to take into account inspection
criterion to investigate the structure periodically to see
whether or not crack have started.
A “damage-tolerant” stool.
The stool has some built in redundancy and is inspected
regularly.
Damage-Tolerant
3- Life Estimation Methods
Fatigue analysis itself usually refers to one of two methodologies.
The stress-life (or S-N method), is commonly referred to as the
total life method since it makes no distinction between initiating or
growing a crack. This was the first fatigue analysis method to be
developed over 100 years ago.
The local-strain or strain-life (E-N) method, commonly referred to
as the crack initiation method, was more recently developed and
is intended to describe only the ‘initiation’ of a crack.
An idealization of the fatigue design process
Total Life Crack Initiation Crack Growth
Fracture specifically describe the growth or propagation of a
crack once it has been initiated and has given rise to many so-
called crack growth methodologies.
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